This photo looks like it could have been taken on another planet - miners harvesting dilithium, perhaps. But this cave full of monstrous crystals is located in Mexico, and the crystals themselves are a form of gypsum, one of the most common elements on Earth. What geologic processes caused the crystals to get so freaking big?The key to the crystals' formation is an upwelling of magma beneath the cave system. The heat caused the anhydrite in trapped pockets of groundwater to form selenite (a form of gypsum), which formed into crystals. Usually, something eventually disrupts this process. There are crystals all over the world, some of them quite large, but none quite as enormous as these. This particular cave remained in total isolation, with a steady temperature from the magma and a static environment in which the crystals were constantly bathed in water. This allowed them to grow for tens of thousands of years. Some crystals are more than 30 feet long and weigh more than 50 tons. In the mid 1980s, the cave was drained (accidentally) by miners, but it wasn't discovered until 2000. Today, Cueva de los Cristales is protected by a steel door, put in place by the mining company that owns the cave. Visitors are limited, but the lack of water means the crystals could eventually collapse under their own weight, or lose their translucent beauty as gases infiltrate the cave. Image by: Carsten Peter, Speleoresearch & Films.Cavern of Crystal Giants. [National Geographic]